The present invention relates generally to communications, and more specifically to a novel and improved method and apparatus for transmit diversity pilot processing.
Wireless communication systems are widely deployed to provide various types of communication such as voice and data. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), or some other modulation techniques. A CDMA system provides certain advantages over other types of systems, including increased system capacity.
A CDMA system may be designed to support one or more CDMA standards such as (1) the xe2x80x9cTIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systemxe2x80x9d (the IS-95 standard), (2) the standard offered by a consortium named xe2x80x9c3rd Generation Partnership Projectxe2x80x9d (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), (3) the standard offered by a consortium named xe2x80x9c3rd Generation Partnership Project 2xe2x80x9d (3GPP2) and embodied in a set of documents including xe2x80x9cC.S0002-A Physical Layer Standard for cdma2000 Spread Spectrum Systems,xe2x80x9d the xe2x80x9cC.S0005-A Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems,xe2x80x9d and the xe2x80x9cC.S0024 cdma2000 High Rate Packet Data Air Interface Specificationxe2x80x9d (the cdma2000 standard), and (4) some other standards.
One technique used to enhance performance, including system capacity and data throughput, is to lower the required transmit signal power by employing transmit diversity. Transmit diversity involves transmitting data on two or more antennas, where the geographical separation between the antennas leads to path loss characteristics that are independent from antenna to antenna. Thus, a receiving station can coherently combine signals from the transmit diversity antennas, and the noise introduced in the channel will not combine coherently, thus increasing the signal-to-noise ratio (SNR) received.
To differentiate between the transmitted signals, a different pilot sequence is used for each antenna. In a two-antenna scheme, for example, a primary antenna transmits a primary pilot sequence, and a diversity antenna transmits a diversity pilot sequence. The diversity pilot sequence can be generated by multiplying the primary pilot sequence by an orthogonalizing sequence, so that the primary and diversity pilots are orthogonal.
In order to demodulate each of the transmit diversity signals in a mobile station, an estimate is made of the pilot signals associated with them. One method for pilot estimation, known in the art, takes advantage of the relationship between the primary and diversity pilot just described. Received samples are despread with the primary pilot sequence and accumulated. A buffer stores the previous accumulated symbol. The current and previous accumulated symbols can be added or subtracted at certain symbol boundaries, according to the relationship between the primary and diversity pilots, to yield an estimate of both the primary and diversity pilot symbols. For example, using the pilot sequences defined in the W-CDMA specification, pilot estimates are generated every other symbol. These symbols are used for data demodulation of each symbol, operating under the assumption that during alternating symbols, the pilot estimates are approximately equal to those received during the prior symbol. As such, during every other symbol, the most recently despread pilot symbol is not factored in the channel estimate, and, therefore, not used in data demodulation. This process is detailed further below.
Under certain circumstances, the assumption that sequential pilot symbols are approximately equal is valid, and hence demodulation using prior pilot symbol estimates is sufficient. However, when channel conditions change more rapidly, such as when a mobile station is traveling at high speeds, that assumption may no longer be valid, and demodulation performance may degrade to a point insufficient for the desired communication performance level. There is therefore a need in the art for improved diversity pilot estimation in rapidly changing channel environments.
Embodiments disclosed herein address the need for improved diversity pilot estimation in rapidly changing channel environments. In one aspect, a difference estimate is updated when a current received symbol comprises a difference between the primary and diversity channels, and a sum estimate is updated when a current received symbol comprises a sum of the primary and diversity channels. The primary channel and the diversity channel are estimated in response to the sum estimate and the difference estimate. In another aspect, the sum estimate and difference estimate are produced by filtering the received symbols. In yet another aspect, received symbols are demodulated in response to the primary and diversity channel estimates. These aspects have the benefit of incorporating the most recently received symbol, resulting in more accurate channel estimates and hence improved demodulation performance, increased system capacity, lower required transmit power, and other benefits.
The invention provides methods and system elements that implement various aspects, embodiments, and features of the invention, as described in further detail below.